For the event 51
https://learn.microsoft.com/en-us/troubleshoot/windows-server/backup-and-storage/event-id-51-information
as to replication you can try a non-authoritative restore. If DFSR
https://support.microsoft.com/en-us/help/2218556/how-to-force-an-authoritative-and-non-authoritative-synchronization-fo
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